The long-term goal of the proposed research is to understand the developmental processes that regulate organ formation. Current work focuses on the radial organization of the Arabidopsis root as a simple and tractable system. A novel signaling pathway has been discovered that links two key players in radial patterning, SHORT-ROOT (SHR)and SCARECROW (SCR). SHR is translated in the stele and then moves to the adjacent cell layer where it activates SCR transcription and initiates endodermal specification. SCR, possibly in concert with SHR,regulates the asymmetric cell division that results in the formation of cortex and endodermis. Preliminary results indicate that SHR interacts directly with the SCR promoter. A meta- analysis of results from genomics approaches identified other candidate targets of SHR that may be involved in cell division and cell specification. Domains and residues within the SHR protein have been identified that are required for movement and activity. Building on this foundation, several hypotheses relating to this central pathway in root development will be tested. A structure/function analysis and yeast two-hybrid approach will be used to test the hypothesis that SHR movement and activity are mediated by interaction of specific domains in the SHR protein with other factors. Genomics approaches will be used to test the second hypothesis that SHR regulates asymmetric cell division and cell specification through a transcription factor cascade. A genetic screen and the yeast one-hybrid method will be used to test the third hypothesis that the SHR-mediated pathway is initiated in embryogenesis and maintained during development by transcription factors that are expressed in a tissue-specific manner. Specific aims are to: 1) Elucidate the mechanisms regulating SHR movement and activity; 2) Determine how the SHR-mediated signaling pathway controls cell specification and asymmetric cell division: 3) Identify the factors that initiate and maintain the SHR-mediated signaling pathway. The picture that is emerging of the SHR-mediated regulation of radial patterning has some clear similarities and apparent differences with pathways regulating animal organogenesis.Comparing and contrasting plant and animal development can provide important insights into both, leading to a better understanding of developmental^ related health concerns such as birth defects and cancer as well as improvements in biotechnology and agriculture.